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PCI Express Extender Cables Benchmarked

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post #1 of 106 (permalink) Old 09-18-2013, 02:55 PM - Thread Starter
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After posting my wall mounted computer which uses PCIe extenders, I've got many questions regarding whether or not the extenders impact performance. So for your viewing pleasure, I have tested multiple extender configurations using my spare system.

The lovely people at 3M was very generous and sent me a sample of their very expensive PCIe extender assemblies! (Same ones I'm using on the wall mount rig)

Lets meet the contenders!



[PCIe Extenders]

[Test System]
  • Processor: Intel Core 2 Quad Q6600 (Stock @ 2.4GHz)
  • GPU: AMD Sapphire HD5870 1GB (Stock @ 850/1250Mhz)
  • Memory: 2x4GB Mushkin Redlines DDR2-800
  • Motherboard: Asus Commando
  • Cooling: Freezer 13 Pro
  • HDD: WD 640GB
  • Power Supply: Corsair TX850M
  • Monitor: LG Flatron W2442PA 1080p
  • OS: Windows 7 Ultimate x64

[Test Software]
  • 3DMark - Cloud Gate
  • 3DMark - Fire Strike
  • Unigine - Heaven
  • Unigine - Tropic

Final score will be the average of 3 benchmark runs.

[Test Setup]

In Slot


3M Extender


300mm + 200mm Extender


300mm Extender


200mm Extender


[Result]




*Cloud Gate score average reduced by 1/3 to better match bar height with rest of the test columns

Only hiccup during testing was during the 300mm + 200mm extender test. The system BSOD at 2nd Heaven benchmark, BCCode: 124

[Conclusion]

Based these benchmark results, I conclude that there is no measurable performance impacts when using PCI express extenders compared to in-slot configuration. In addition, there is no measurable performance impacts between different types and lengths of extenders used.

The shielded cable and unshielded cable was essentially identically in performance in this setup. However, 1 incident of BSOD was observed when using the unshielded cable while testing.

[Remarks]

The unshielded extenders worked quite well in these tests. However, they will cause considerable trouble if they are near EMI sources or have additional riser cables on top of the other.
I fielded the question of why the regular cables didn't work for me in a dual GPU configuration and here is their response:
Quote:
There are a number of contributors to problems. The first would be that with the cables that you are using, they are not impedance matched, and may not be low loss dielectric. Thus, attenuation, cross talk and reflections will close the eyes, due to loss and noise. Reflections are the result of impedance discontinuities. Our assemblies are tuned to 85 ohms, cable and PCB. PCIe Gen3 is 85 ohms. 85 and 100 ohms work interchangeably, but if mixed, you lose some signal to reflection and shorten the maximum possible length. If your performance is on the edge, that's not a good thing. Our cable is tightly coupled and uses silver plated conductors. Lower loss for conductivity, and for radiative losses coupling to ground. And we use a low loss dielectric. Impedance matching is one thing, but dielectrics absorb some of the energy too. The only thing separating these assemblies from what is used in supercomputers is the connector, and we have no control over that. The connectors are geared at being cheap and good enough, by Intel and the industry. By and large, they are.

As for crosstalk, our cable exhibits less than -40 db of crosstalk. Crosstalk can occur at the termination, since the shielding has been removed. We design the PCB termination and stackup to minimize the effects. Again, the other cable is not shielded, so there could be considerable cross talk along it's length. Between attenuation, crosstalk, and reflections there may be no eye at the far end. So for dual GPU, I think it is simply a cable performance issue. The other cable assembly was probably designed with PCIe Gen I in mind. I have seen some that advertise Gen 2, but the longest that I recall there was around 6 inches. And the prices were considerably higher than Gen I.

If you'd like to find out more on why extenders doesn't affect performance, there is a good discussion thread on Reddit regarding this issue.
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post #2 of 106 (permalink) Old 09-18-2013, 03:19 PM
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Although you didn't get much variation it's still very useful information!
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post #3 of 106 (permalink) Old 09-18-2013, 03:36 PM
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Very surprising and helpful results. +1 rep. Love the build too.
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post #4 of 106 (permalink) Old 09-18-2013, 03:43 PM
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Not the results I expected but good to know. If you can do one think in the results chart below the graph, bold the best results in each test to help see it better as its tough to read through the numbers. Thanx!

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post #5 of 106 (permalink) Old 09-19-2013, 05:02 AM
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I suspect the only difference is EMI shielding then ?

Or are the cables 30 AWG silver?

Silver resistance is1.59 X10−8 Ω·m @ 20°C
Copper resistance is 1.68 X10−8 Ω·m @ 20°C

There's not much difference

I think some manufacturers make flat cable EMI shields such as "Parker Chomerics CHO-JAC® CJ-021 Flat Cable EMI Shield "
Site: http://www.chomerics.com/products/emi/cableconnector/cable/foil/cho-jac/index.html
DATA: http://www.matweb.com/search/datasheet.aspx?matguid=f4df0722c58a48d986944987334b1f47&ckck=1

and this one from SPC , not for flat cables
http://www.newark.com/pro-power-formerly-from-spc/spc5086/heat-shrink-tubing-kit-emi-shield/dp/50N161

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post #6 of 106 (permalink) Old 09-20-2013, 11:33 AM
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Thanks for this thread. Im going to be using a riser cable for my build to mount a graphics card vertically in my case. I already bought a 12" riser cable off ebay, its one thats soldiered with bus cables like the ones you daisy chained. I only plan on using one GPU in my build, just one bad ass card most likely getting a 790 gtx when they drop. I hope using one will work, if not ill buy the 100$ cable. thanks for the information and the post.

Awesome build by the way, very unique
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post #7 of 106 (permalink) Old 10-07-2013, 06:26 AM
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Just the information I was looking for! thumb.gif

I bought some of the generic risers without the shielding on eBay for less than $10 each.
My plan was to use them to move an SLI configuration to a location with better airflow.
After testing the configuration with the risers, I decided that the risers were garbage and trashed them.
I'm really glad to know that it may have just been the fact that I had an unshielded riser resting on another unshielded riser.
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post #8 of 106 (permalink) Old 10-07-2013, 06:37 AM
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So iam just curious why would one use these extender cables? Just when you would wanna mount your gpu out of site or something along those lines? Ive never seen these before,Interesting! thumb.gif

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post #9 of 106 (permalink) Old 10-07-2013, 09:27 AM
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Quote:
Originally Posted by nz3777 View Post

So iam just curious why would one use these extender cables? Just when you would wanna mount your gpu out of site or something along those lines? Ive never seen these before,Interesting! thumb.gif

You should check out the OP's build. That will explain it perfectly. There are some of us who think outside the box.
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post #10 of 106 (permalink) Old 10-07-2013, 09:31 AM
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We need inovators like that, I think I saw the wall-Monunted pc,Is that the one you guys are talking about? Its good to know theres no performance hit using these guys, I thought on account of the Length itself we might see a hit, but thats really good for future refrence.

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